Methods for manufacturing thermoplastic polyesters, in particular high-molecular-weight polyesters—for fibers, sheets, bottles and packaging material, are known.
The starting compounds, can be dicarboxylic acids or their esters and bifunctional alcohols that are converted in a melt. In a first reaction step, these are esterified or transesterified into monoesters, diesters, and oligoesters. The next process step is a prepolycondensation, in which under reduced pressure under elimination of condensation products such as water and alcohol, longer chains are built up. The viscosity of the melt gradually increases during this process step. This process step is followed by a polycondensation, in which a further chain build-up occurs with a distinct viscosity increase under greatly reduced pressure (vacuum).
In both the precondensation and the polycondensation it is important that the low-molecular-weight condensation products that are formed and the released monomers be removed as fast as possible from the reaction mixture, i.e. pass quickly from the liquid phase to the gas phase.
In principle, this can be achieved, depending upon the viscosity, by stirring and/or a selective increase in surface area with simultaneous reduction of the pressure up to vacuum. For the precondensation step, for example, in which considerable amounts of condensate still accumulate, agitated vessels are predominantly employed since the viscosity is still relatively low. An additional thorough mixing occurs through the outgassing of low-molecular-weight products, and agitated vessels are also among the standard equipment for vacuum processes.
Nevertheless, there has not been a lack of attempts to replace agitated vessels with other devices, in order to avoid their disadvantages such as longer diffusion paths due to an unfavorable surface/volume ratio, danger of dead-space formation and non-uniform residence times, i.e. a broad residence time spectrum, and in order to eliminate potential sources of failure such as e.g. agitator shaft seals.
EP 0,000,918 describes a method for the polycondensation of polyesters in a thin layer. There, a precondensation product is heated to 270°-340° C. in a heat exchanger in order to obtain a thin layer with a corresponding diffusion, and polycondensed in an adiabatic operation under vacuum (1.33 mbar), the temperature being continuously lowered by 30°-50° C.
For the realization of thin layers, pipes with internals such as heated inclined surface areas or heated pipes of different diameters are proposed. The high control complexity for the heating of the pipes in order to maintain the required temperature gradient, to ensure the intended film formation in all places, and the possibility of the formation of dead spaces are unfavorable. Additionally, nothing is said about how the conditions would be sustained under production conditions.
U.S. Pat. No. 4,973,655, a large surface is generated for the polycondensation by spraying a precondensate into an evaporation reactor. This leads to a polycondensation in solid phase, where the product thus formed is sprayed immediately into an extruder for further processing. This method, mainly intended for the manufacturing of polyamides, has also been proposed for polyester processes, without substantiating it by examples. There is nothing known about such an application.
U.S. Pat. No. 2,727,882 describes a tray column for the precondensation step of the polyethylene terephthalate process with specially shaped outlets, in which the product from the transesterification step is directed from the bottom up and the pressure is reduced from tray to tray with simultaneous increase of the temperature. The initial pressure is about 30-130 mbar, the final pressure at the head is 13 mbar, the temperature increase is between 5° C. and 25° C. It is explicitly required that the intrinsic viscosity (IV) not be greater than 0.3. That is, the melt has to be relatively low-viscosity in order to guarantee the functioning of this column. A somewhat higher viscosity will lead to less thorough mixing, to an inferior outgassing of mono- and oligomers, and to longer residence times that are undesired.
This limits the application possibilities for copolyesters and other polyesters. There is nothing said about the reprocessing of the vapors.
The column described in U.S. Pat. No. 2,727,882 is operated from the bottom up. Therefore it is very susceptible to failure, e.g. with throughput variations. In case of failure, the column empties very quickly and has to be restarted after remediation of the failure. Such failures have a large impact on the operation of the downstream end reactor and the product quality.
In EP 346,735, a column with internals for the precondensation is employed that is characterized by a large length/diameter ratio (133:1 to 80:1). The very small cross-section of 50 mm given leaves doubts about an implementation in large-scale installations. The flow through occurs top down, the residence times mentioned are extremely short (<10 min), the pressure is 500-800 mbar at the entrance, 13-0.7 mbar at the exit, temperatures of 260°-320° C. are mentioned. The internals mentioned in the document are tower packings such as Raschig or Pall rings that should have a free surface of 0.9-1.5 m2/L in the first third and a surface of 0.3-0.5 m2/L in the two other thirds. The vapors liberated in the separator are distilled and the diols returned to the process. The precondensate obtained that way is further condensed in the next step until a granulatable product is obtained, i.e. in principle a second precondensation, or a polycondensation to high molecular polyesters follows.
The equipment described in the documents U.S. Pat. No. 2,727,882 and EP 346,735 are tray or packed columns that have the advantages and disadvantages that are typical for their column type. As also especially emphasized in the patent, the tray column is characterized by good exchange on the individual trays, but both the longer residence time and the limiting viscosity are very unfavorable, especially when copolyesters or other polyesters such as e.g. PTT or PBT are to be manufactured. There is the danger that due to longer residence times, side reactions such as the formation of allyl alcohol and acrolein with PTT (polytrimethyleneterephthalate) or the formation of tetrahydrofuran with PBT are (polybutyleneterephthalate) facilitated. Additionally, a noticeably increased amount of diol is required for the operation of the tray column from the bottom up. This is very unfavorable energetically and increases the formation of side products. The packed column does have the advantages of a shorter residence time, but a transfer of the sizes given in the document to large-scale processes will be difficult.
In summary, it can be determined that the solutions proposed in the documents given above do not represent or represent only somewhat convincing alternatives to the conventional, versatilely applicable, and technically robust agitated vessel.